1. Field of the Invention
The present invention relates to subterranean filters for use in oil, gas, and water wells. More particularly, the invention relates to filters having a non-woven, polymer membrane surrounded by a metal braided layer giving the filter improved permeability, resistance to chemical breakdown, and physical strength.
2. Background of the Related Art
The problem of reliably removing particulates from liquids or gasses (production fluids) exist in many types of wells including oil and gas wells, water wells, geothermal wells, and wells for ground remediation. Typical particulates needing to be filtered out are sand and clay including unconsolidated particulate matter, also known as xe2x80x9cformation sandxe2x80x9d. A major problem in producing hydrocarbon fluids from unconsolidated formations is the intrusion of formation sand, which is typically very fine, into the production fluid and equipment. The presence of sand in the production fluid often leads to the rapid erosion of expensive well machinery and hardware.
Subterranean filters, also known as xe2x80x9csand screensxe2x80x9d have been used in the petroleum industry for years to remove particulates from production fluids. They generally comprise a perforated inner member or pipe, a porous filter membrane wrapped around and secured to the outer periphery of the pipe and an outer cover. Filtering in a subterranean well is typically performed at the position in the well where the fluid enters the production string. A common way to achieve the filtration is to mount a tubular filter in the production string near the area of fluid production such that the produced fluid must pass through the filter prior to entering the production string and being pumped to the surface.
FIG. 1 is a sectional view showing the position of a filter in use with a submersible rotary pump. Well 10 comprises well head 11, casing 12, and production string 13. Production string 13 comprises piping 16, submersible pump 17, filter 18, and electric motor 19 for driving the pump. Filter 18 removes particulate matter which may otherwise cause damage to pump 17 and other equipment used in the production and collection process. The filter 18 has threads at each end for attaching the filter to adjacent members of the production string. Pump 17 and motor 19 are also threadedly attached to the production piping. Filter 18 has perforations 21 exposing the interior of the production piping 16 to production fluid in well annulus 22. Casing 12 is perforated 26 exposing annular space 22 to production fluid present in production zone 27. The perforation extends outward into zone 27 as illustrated by fracture 25. Production fluid flows from zone 27 through perforations 26 into annular space 22, through filter perforations 21, through pump 17, into piping 16, and to wellhead 11 for collection.
Various filter designs using various membrane materials are currently used to filter sand from unconsolidated formations. Membrane materials include metal screens, sintered fibers, ceramic materials, woven polymer fabrics and Dutch twill weaves. All of these prior art filters are subject to failure from chemical and physical forces present in a well. For example, oil and gas wells are often treated with enhanced recovery chemicals (stimulation chemicals) which are often highly corrosive. In addition, corrosive acids may also be naturally present in crude oil or gas. Filters having metallic membranes, such as sintered or wire screen membranes, are subject to failure due to corrosion. Filters with ceramic or Dutch twill membranes are also susceptible to chemical damage.
In addition to chemical threats, filters can be damaged or destroyed by the extremely high hydrostatic pressures at which they sometimes operate. These pressures result in high stresses in the radial direction, which may cause the filter to deform or collapse. A deformed filter may effectively close areas of the filter membrane which may cause sediments to accumulate in, and ultimately clog the filter. In filters having ceramic or Dutch twill membranes, the membrane material itself may fail in the radial direction under high pressures.
Another widely used enhanced recovery technique in oil and gas wells that can damage a filter is the use of gravel packing. To prevent the perforations in a well casing wall from filling in with sediment, which will block the flow of fluid, sand is pumped into the well to fill the perforations and the annular space between the filter and the well casing. The well shown in FIG. 1 has been gravel packed. The sand or gravel has a large enough grain size such that the fluid will flow through the sand packing and into the well. The sand must be pumped under very high pressure down the well bore and into the perforated formation. This high-pressure environment may cause the filter near the packed formation to deform or collapse.
Yet another problem encountered by submersible filters are highly deviated wells.
Oil and gas wells are often directionally drilled to increase the length of production pipe in the formation production zone. In fact, some wells may be first drilled vertically and then transition across a ninety-degree angle to a horizontal bore. This requires that the production string, including the filter section, have sufficient flexibility to bend around these deviations as the string is lowered into the well. Filters having metallic filter membranes may buckle or tear if the bend is too severe. Filters having ceramic or ceramic based filter membranes tend to be brittle and may crack as they deform. Dutch twill weaves also can be damaged in bending through deviated wells.
It is frequently impossible to completely prevent damage to a filter. In fact, when an underground formation collapses or shifts, it is not uncommon for a filter surrounded by the formation to undergo substantial deformation, such as elongation or crushing. Ideally, a well filter should be able to experience large deformations without losing its ability to prevent the passage of particulate matter, but conventional well filters typically suffer a severe drop in filtering ability even when subjected to even modest deformation.
In addition to their susceptibility to damage from chemical and physical abuse, membrane materials of the current types used all exhibit limitations in permeability as measured by a pressure drop across the filter during use. Filter failure, whether it is from physical or chemical damage or from the filter element""s lack of permeability, requires removing the production string, installing a new filter, and lowering the string back into the well a costly and time consuming procedure.
Therefore, a need exists for a subterranean well filter that can more effectively separate particles from production fluids.
There is a further need for a subterranean filter that is resistant to physical and chemical damage and can retain its filtering capabilities if damaged.
The present invention provides a subterranean well filter for the removal of particulate matter thorough the use of a filter formed of a non-woven polymer material and strengthened with a layer of braided metal. The filter not only blocks the passage of particulate matter, it is also highly resistant to corrosion and highly flexible, giving it strength to resist damage due to physical stress or chemical exposure.
In one aspect of the invention, a subterranean filter comprises an inner member having a perforated wall permeable to fluid flow and defining a flow passage through which fluid may flow upward to the wellhead. A filter membrane material is disposed around the inner member and blocks the flow of particulate matter of a predetermined size into the flow passage of the inner member. The filter membrane comprises a single layer or multiple layers of a non-woven polymer. A metal braided layer is integrally formed around the membrane and the inner member adding strength and flexibility to the filter.
In another aspect of the invention, the inner member is made of perforated, coiled tubing having a filter membrane and a metal braided layer disposed therearound.